Punches are typically used for perforating sheet stock, such as sheet metals, plastics, leather and similar sheet articles, in processes of forming the sheet stock into various manufactured items. Perforating is accomplished by reciprocating the punches downwardly through the sheet stock over a die opening formed to match the punch. After perforating the punch reciprocates upwardly. After punching the sheet stock tends to stick around the punch and cling to the punch. If the stock clings to the punch the punching operation and the flow of the workpiece through the punch are obstructed.
Typically, punches are provided with a device called a stripper, which clears the punch by forcing the sheet stock away after the punch has reciprocated upwardly after striking and punching the hole. In the past, strippers were made of two parallel plates, concentric about the axis of the punch with the top plate being the head plate and the bottom plate being the stripper plate. A steel spring mechanism was placed between the two plates and held together with shoulder bolts, allowing the distance between the plates to be reduced by the pressure action of the punch and allowing the punch to pass through a hole in the stripper plate to perforate the sheet stock placed below the stripper plate. As the punch reciprocated upward, the spring mechanism forced the stripper plate downwardly, pushing the sheet stock away from the punch. The steel spring mechanism had a number of disadvantages in that it was costly to manufacture, the springs would weaken with age and the travel of the stripper was limited by the spring mechanism itself.
More recently, a stripper mechanism has been used which consists of a circular head plate and a circular stripper plate which are bonded to a cylindrical resilient elastomer material. This bonded mechanism is less costly to manufacture than the previous devices, but it also has a number of disadvantages. One difficulty is that the cylindrical shape of the elastomeric material evidently creates stresses in the material which eventually lead to failure due to fatigue and rupture of the elastomeric material. In addition, the bonded structure requires that the entire stripper mechanism be discarded and replaced with a new one when the elastomer fails. There is no mechanism provided whereby the elastomeric material only can be readily replaced. Further, the elastomer device and the steel spring devices require that a large variety of strippers must be kept in inventory, since each stripper will operate effectively for only one predetermined punch diameter.
I have devised a new stripper mechanism which eliminates the defects in prior devices by utilizing a resilient elastomeric material as the spring. The elastomeric material is formed in a prism of rectangular or square cross section. This shape appears to eliminate or reduce the cast in stresses in the elastomeric material with the result that the material has a much longer life and does not fail as readily due to rupture from fatigue. In addition, by using a rectangular or square cross section, more of the elastomeric material may be used in a given space on the punch apparatus so that greater endurance is achieved. By being able to introduce a larger volume of elastomeric material into the stripper, less compression force per unit volume is required to clear the punch of the sheet stock. As a result the elastomeric material is fatigued less.
In addition, I have provided a structure which allows the head plate, stripper plate or the elastomeric spring to be readily interchanged. As a result, considerable savings are achieved since, on failure of the elastomeric material, a new elastomeric part can be readily substituted without replacing the entire stripper mechanism.
Considerable savings are also achieved in required inventory. One elastomeric spring is adaptable for use in stripper mechanisms which will cooperate with a number of punches of varying diameters. It is not necessary to carry as great a spare parts inventory of different sizes of strippers in order to have the flexibility required in a varied punching operation or shop. Therefore, less capital is tied up in inventory and less space is required in storing parts.